GROWTH INCREMENTS OF THE BARNACLE BALANUS AMPHITRITE AMPHITRITE DARWIN (CIRRIPEDIA)

The microstructure and mineral growth increments of juvenile barnacles have been studied by scanning electron microscopy. Specimens were reared in the laboratory under uniform conditions of day-length and temperature and were provided with food in amounts that were not growth-limiting Growth increments were produced synchronously in the wall plates, scuta, and alae. In scuta, growth involved the deposition of many linear units, minor ridges, within a single growth increment. The size and number of scutal growth increments of 25-day-old barnacles raised under identical culture conditions showed variability. Increments of the radius were composed of a layer of parallel needlelike crystals. By growing laterally, the crystals of the radius strengthen the junction between the wall plates. The period required to form an increment in the scuta was 1.3-1.8 days and the number of increments increased linearly with age. It appears that a mechanism(s) other than environmental rhythmicity is responsible for incremental skeletal growth in barnacles that have experienced only constant culture conditions.